I believe most of this is up to the typesetter. There are some conventions, so that I'd expect προστίθημι to not have stigma — it might even be seen as προςτίθημι — but I've never come across any reasoning behind the ligature patterns except for speed and to save parchment. I'm basing this on several readings of Thompson's Palaeography book. It wouldn't surprise me if some printers had particular habits.